CN107112929B

CN107112929B - Drive train and method for operating a drive train

Info

Publication number: CN107112929B
Application number: CN201580070130.5A
Authority: CN
Inventors: 格拉尔德·黑亨贝格尔
Original assignee: Ge LaerdeHeihengbeigeer
Current assignee: SET SUSTAINABLE ENERGY
Priority date: 2014-12-22
Filing date: 2015-12-22
Publication date: 2020-12-22
Anticipated expiration: 2035-12-22
Also published as: AT14813U1; US20170343082A1; US10458524B2; CN107112929A; WO2016100991A1; EP3238337B1; EP3238337A1

Abstract

The invention relates to a transmission system comprising a drive shaft (2), a drive (4) and a planetary transmission (3) having three drives or slaves, wherein one slave is connected to the drive shaft (2), one drive is connected to the drive (4) and a second drive is connected to an electrical differential drive (5), wherein the differential drive (5) can be switched directly to a power grid (12) without a frequency converter, so that the transmission system can be operated without a frequency converter.

Description

Drive train and method for operating a drive train

Technical Field

The invention relates to a transmission system comprising a drive shaft, a drive and a differential having three drives or outputs, wherein one output is connected to the drive shaft, one drive is connected to the drive and the second drive is connected to an electrical differential drive.

The invention further relates to a method for operating a drive train comprising a drive shaft, a drive and a differential having three drives or slaves, wherein one slave is connected to the drive shaft, one drive is connected to the drive, and the second drive is connected to an electrical differential drive.

Background

Highly efficient variable-speed operation is often required for work machines, for example for conveying devices (such as pumps, compressors and fans) or for example for grinders, crushers, vehicles or for energy generation devices.

Furthermore, an electric machine is considered as an example of a drive device used in this connection, but the principle is applicable to all possible types of drive devices, such as internal combustion engines.

The electric drives most often used today are three-phase machines, such as asynchronous motors and synchronous motors. In order to be able to operate the electric drive with variable rotational speed, the electric drive is connected to the mains in conjunction with a frequency converter. Thus, although a variable speed operation of the drive can be achieved, this solution is expensive and is associated with significant efficiency losses.

A relatively lower cost and also better efficiency alternative is to use a differential system, for example according to document AT 507394 a. The differential system is characterized by a differential, which in a simple embodiment is a simple planetary transmission stage with three drives or output elements, wherein one output element is connected to a drive shaft of the work machine, a first drive element is connected to the drive, and a second drive element is connected to the differential drive. The drive can therefore be operated with a constant rotational speed of the drive, in a rotational speed-variable manner: the variable speed differential drive compensates for the resulting difference in rotational speed. The variable speed differential drive is usually a three-phase motor which is small compared to the drive and which is connected to the mains by means of a correspondingly small frequency converter.

However, frequency converters are more prone to failure than motors and have a significantly shorter service life.

Disclosure of Invention

The object of the present invention is therefore to provide a device and a method of the type mentioned at the outset, with which the drive train can be operated without a frequency converter.

This object is achieved by a transmission system.

In addition, this object is achieved in a method for operating a drive system.

When the differential drive can be selectively connected to the power supply system via the frequency converter or a line connected in parallel with the frequency converter, the advantage according to the invention is that the drive train can be operated via the frequency converter, as is known per se, but even if the frequency converter has a defect or a malfunction, the drive train can continue to operate, even if its rotational speed variability is eliminated or limited.

Drawings

Preferred embodiments of the present invention are explained below with reference to the drawings. In the drawings:

fig. 1 shows the principle of a differential system for a drive of a pump according to the prior art;

fig. 2 shows a first embodiment according to the invention of a differential system;

FIG. 3 shows another embodiment according to the invention of a differential system;

FIG. 4 shows the speed and power parameters of the differential system of the pump according to the invention; and

fig. 5 shows a characteristic curve of a three-phase motor.

Detailed Description

Fig. 1 illustrates the principle of a differential system for a transmission system with a pump as an example. The work machine 1 is here a rotor of a pump, which is driven by a drive 4 via a drive shaft 2 and a differential 3. The drive 4 is preferably a medium-voltage three-phase motor which is connected to a power network 12, which in the example shown is based on the medium-voltage three-phase motor being a medium-voltage power network. The voltage level selected is, however, dependent on the use case and in particular on the power stage of the drive device 4 and can have any desired voltage level without affecting the basic function of the system according to the invention. The design-specific operating speed range is determined by the number of pole pairs of the drive 4. The operating speed range is a speed range in which the drive 4 can provide a defined or desired or required torque or, in the case of an electric drive, can be synchronized with the power grid 12. The planet gear carrier 7 is connected to the drive shaft 2, the drive 4 to the ring gear 8 and the sun gear 9 of the differential 3 to the differential drive 5. In this embodiment, the core of the differential system is therefore a simple planetary gear train with three drives or outputs, one output being connected to the drive shaft 2 of the work machine 1, one first drive being connected to the drive 4 and one second drive being connected to the differential drive 5.

The drive train therefore essentially comprises a drive 4, a differential 3 and a differential drive 5.

In order to be able to optimally adapt the rotational speed range of the differential drive 5, an adaptation of the transmission 10 between the sun gear 9 and the differential drive 5 is carried out. As an alternative to the spur gear stages shown, the transmission 10 can also be multi-stage or designed as a toothed belt or a drive train and/or as a planetary gear stage or as a bevel gear. Furthermore, with the matching gearbox 10, an axial offset for the differential drive 5 can be achieved, which enables a simple embodiment of the differential drive 5 on the basis of the coaxial arrangement of the work machine 1 and the drive 4. The differential drive 5 is connected to a motor brake 13, which brakes the differential drive 5 when required. The differential drive 5 is electrically coupled to the grid 12 by means of a preferably low-voltage inverter comprising (depending on the type of operation as motor or generator) a rectifier or converter 6a on the differential drive side and a grid-side converter or rectifier 6b, and a transformer 11. The transformer compensates for any voltage differences between the power supply system 12 and the system-side converter or rectifier 6b and can be omitted if the voltage between the drive 4, the system-side converter or rectifier 6b and the power supply system 12 is identical. The rectifier or converter 6a and the converter or rectifier 6b are connected via a dc intermediate circuit and can be separated locally if required, wherein preferably the differential drive-side rectifier or converter 6a is positioned as close as possible to the differential drive 5.

In order to achieve a high reliability of the overall system, the frequency converter may also be constructed redundantly, as proposed, for example, in document WO 2012/001138A.

The main advantage of the differential system is that the drive 4 can be connected directly (i.e. without power electronics) to the power grid 12, as long as the drive is an electric motor. The compensation between the variable rotor speed and the fixed speed of the network-connected drive 4 is effected by means of a variable-speed differential drive 5.

The torque equation for a differential system is:

the torque of the differential drive is equal to the torque of the drive shaft y/x,

the parameter factor y/x is the magnitude of the gear ratio used in the differential 3 and in the matching transmission 10. The power of the differential drive 5 is substantially proportional to the percentage deviation of the pump speed from its basic speed x multiplied by the power of the drive shaft. Correspondingly, a large rotational speed range requires in principle a correspondingly large size of the differential drive 5. This is why differential systems are particularly well suited for small rotational speed ranges, but in principle any rotational speed range can be achieved.

Fig. 2 shows an embodiment of the differential system according to the invention. The illustrated drive train here, as in fig. 1, also has a drive 4, a differential 3, a differential drive 5 and

frequency converters

6a, 6b, which are connected to an electrical network 12 by means of a transformer 11. The work machine 1 is also driven here by means of a drive shaft 2.

The transformer compensates for the voltage difference existing between the power supply system 12 and the differential drive 5 if necessary and can be omitted if the voltages are identical.

The motor brake 14 is located in this embodiment variant (alternative to the position of the motor brake 13 in fig. 1) between the differential drive 5 and the sun gear 9. The motor brake is connected to the gear shaft of the mating transmission 10 by way of example, but can in principle be arranged at any desired point between the sun gear 9 and the differential drive 5, depending on the constructional requirements. Thus, the differential drive 5 can be disassembled for possible required repairs and the differential system still continues to operate at the basic rotational speed "T" (fig. 4).

As an alternative to the motor brakes 13 and/or 14, any type of force-locking and/or form-locking or blocking can be provided. The locking is either set up standardised or used when required.

However, as already mentioned at the outset, the

frequency converters

6a, 6b are more prone to faults and have a significantly shorter service life than the electric motors. For this reason, a solution is important for the user of the device: this solution enables continuation of the operation as soon as the frequency converter is not (any longer) ready for operation.

According to the invention, this object is achieved in that, in the event of a failure of the

frequency converters

6a, 6b, the frequency converters are disconnected from the differential drive 5 and the differential drive 5 is connected directly (or if necessary via the transformer 11) to the electrical grid 12 via the lines 15. For this purpose, two

switches

16, 17 are provided, with which the differential drive 5 can be switched alternatively to the

frequency converters

6a, 6b or to the line 15. Thus, at least one operating point with a fixed rotational speed can be set.

If the differential drive 5 is designed as a pole-switchable three-phase motor, at least two, if necessary also a plurality of synchronous rotational speeds can be achieved by: two or more electrically separate windings are mounted in the stator of a three-phase electrical machine. Typically 4 and 6 or 4 and 8 pairs of electrodes. Thus, for example, in a 50Hz network a 4-pole three-phase machine has a synchronous speed of 15001/min, a 6-pole three-phase machine has a synchronous speed of 10001/min, and an 8-pole three-phase machine has a synchronous speed of 7501/min.

Thus, two or more operating points for a work machine (1) with a fixed rotational speed can be realized according to the implemented pole changing possibilities. However, even if the differential drive 5 is not pole-shiftable, the drive train always continues to run at the (one of the) synchronous rotational speed.

The other fixed-speed operating point for the work machine (1) is at the base speed "T". This basic rotational speed is the operating point at which the sun wheel (9) is located, for example, when the

brakes

13, 14 are activated.

Fig. 3 shows another embodiment of the differential system according to the invention. In principle, the differential system is constructed identically to that shown in fig. 2. In a development of the system according to the invention, the differential drive 5, which is designed as a three-phase motor, can be connected to the electrical network 12 with different directions of the rotating magnetic field.

In order to change the direction of rotation, the direction of rotation of the standard rotating magnetic field must be changed. In this case, the exchange of two outer conductors, for example the outer conductors L1 and L3, is sufficient when the three-phase system is operated. For this reason, a common circuit is a commutation protection circuit.

In practical applications, the motor terminals U2, V2, and W2 bridge and are in the rotating magnetic field on the right: l1 is switched on to U1, L2 is switched on to V1 and L3 is switched on to W1, or in a rotating magnetic field on the left: l1 switches on W1, L2 switches on V1 and L3 switches on U1. By changing the rotating magnetic field, the differential drive 5 operates either as a motor (power flow direction "a") or as a generator (power flow direction "b"). Thus, a further operating point with a fixed rotational speed is obtained for work machine 1. If the three-phase machine is pole-changeable, a plurality of additional operating points with fixed rotational speed are correspondingly obtained.

In the embodiment according to the invention of fig. 3, the line 15 symbolizes the rotating magnetic field with which the differential drive 5 is rotated into the range of the motor and thus in the power flow direction "a". A further line 18, which can be connected to the differential drive 5 by means of a switch 19, symbolizes the rotating magnetic field with which the differential drive 5 rotates into the range of the generator and thus in the power flow direction "b". Of course, in practice, it is also possible that only one of the two

lines

15, 18 is present and, for example, the outer conductor is connected as described for changing the direction of rotation.

In the differential system according to fig. 3, at least three operating points with a fixed rotational speed can be realized, and at least five operating points can be realized in the case of a three-phase electric machine with pole-changing. In a simplified embodiment of the differential system, the

frequency converters

6a, 6b can be omitted and the work machine 1 can be operated at a plurality of operating points with a fixed rotational speed.

Fig. 1 to 3 show a differential system in which a first drive is connected to a ring gear, a driven is connected to a planet carrier, and a second drive is connected to a sun gear. In a further variant according to the invention, the second drive can be connected to the planet carrier 7, the first drive to the ring gear 8 and the output to the sun gear 9. Other alternative combinations are also encompassed by the invention.

Fig. 4 shows the speed and power parameters of a differential system according to the invention, for example for a pump. The diagram shows the power value and the rotational speed value for the pump as work machine 1, drive device 4 and differential drive 5 each being plotted against the rotational speed value of drive shaft 2 ("pump rotational speed"). The drive 4 is connected to the power supply system 12 and the rotational speed of the drive ("motor speed") is therefore constant, in the example shown for a four-pole three-phase machine in a 50Hz power supply system approximately 15001/min. The operating speed range for the drive shaft 2 is 68% to 100%, wherein 100% is the selected nominal or maximum point of the differential system. The rotational speed of the differential drive 5 ("servo rotational speed") is in this case-20001/min to 15001/min, depending on the transmission ratio of the differential system, wherein a rotational speed of approximately 15001/min is the synchronous rotational speed of the differential drive 5 selected for the example (in the example shown, likewise a 4-pole three-phase motor in a 50Hz network). Approximately at this synchronous speed, the differential drive 5 provides the nominal torque. The rated torque is to be understood here as the maximum torque which the three-phase machine can continuously provide under the given environmental conditions.

Fig. 4 shows that the differential drive 5 operates as a generator (-) and as a motor (+). Since the maximum required power of the differential drive 5 in the range of generator (-) is less (approximately 110kW) than the maximum required power in the range of motor (+) (approximately 160kW), the differential drive 5 can be operated continuously in the range of generator (-) in the so-called demagnetization range, whereby higher rotational speeds (above its synchronous rotational speed) can be achieved for the differential drive 5 (however with reduced torque). The rotational speed range for the work machine 1 can thus be extended in a simple manner to the operating point "C". If the differential system according to fig. 3 is configured, the operating points "a" and "B" can be reached by the change of the rotating magnetic field. That is, the differential system can realize an operating point over almost the entire operating rotational speed range of the work machine 1 with the differential drive 5 according to the invention directly connected to the electrical grid 12 (corresponding to the embodiment according to the invention as depicted in fig. 3 without the

frequency converters

6a, 6 b).

In a particularly simple embodiment of the differential system, the frequency converter can be designed as a so-called 2Q system (two-quadrant frequency converter), whereby the differential system is then only designed for the range of the motor (+) type. Therefore, the grid-side rectifier 6b can also be configured as a simple diode rectifier, for example. In this embodiment of the differential system, the work machine 1 can also be operated both at its minimum and at its maximum with the differential drive 5 according to the invention directly connected to the electrical grid 12 (corresponding to the embodiment according to the invention described in fig. 2 without the

frequency converters

6a, 6 b).

If the differential drive 5 is designed as a pole-changeable three-phase motor, a rotational speed between a minimum rotational speed and a maximum rotational speed can also be achieved by a corresponding switching of the pole pair numbers.

In fig. 4, the point "T" indicates the so-called "base rotational speed" of the drive shaft 2, at which the rotational speed of the drive 5 is equal to zero. In an ideal manner, point "T" lies in an operating range in which the device is operated for a large time portion. In this operating point, the

motor brakes

13, 14 can be activated, whereby the differential drive 5 does not have to be operated and the losses and losses associated therewith are therefore avoided. In the range of the motor type (+) of the characteristic curve, the drive device 4 and the differential driver 5 are driven in parallel. The sum of the two powers is the drive power for the drive shaft 2 after deduction of the accumulated system losses ("system power"). In the generator range (-), the drive 4 must compensate for the power of the differential drive 5 ("servo power"), whereby the total system power ("system power") is the drive power of the drive 4 ("motor power") after deducting the power of the differential drive 5. That is, the efficiency is higher in the motor (+) range.

In principle, it is determined that: the smaller the power flow through the differential drive 5 and thus the higher the overall efficiency of the system, the closer the pump speed ("pump speed") is to the basic speed "T". However, since the required drive power also increases with increasing pump speed, the required size of the drive means 4 can be reduced by the size of the differential drive 5 by parallel driving of the drive means 4 and the differential drive 5 compared to drives according to the prior art.

It goes without saying that the individual described measures for stopping or operating the differential drive 5 in the described operating points can be used either individually or in any combination with one another, so that depending on the use case at least one operating point can also realize any desired number of operating points for the drive train, even if the frequency converter or even the differential drive is not operated.

During operation of the transmission system, it is also possible to change between the described operating points, so that variable-speed operation is possible. In the operating points, the differential drives 5 (if they are in operation) are each operated at different rotational speeds, but the rotational speeds are each synchronous rotational speeds, since the differential drives are connected to the electrical grid 12 directly or, if necessary, only via the transformer 11.

As work machine 1, a pump is shown in fig. 1 to 3 in an exemplary manner. However, the concepts described here can also be used in drives for all other types of work machines, such as compressors, fans and conveyor belts, grinding mills, crushers, etc., or energy generating devices and the like.

In the case of the use of the system according to the invention in an energy production plant, the drive 4 is operated essentially in generator mode operation and thus reverses the power flow in the entire drive train.

If the differential system according to the invention with the differential drive 5 which can be connected directly (without the

frequency converters

6a, 6b) to the electrical grid 12 is powered up, the drive 4 is preferably first switched on to the electrical grid, while the second drive (sun wheel shaft 9 or differential drive 5) is preferably held braked by means of the

service brakes

13, 14 or by means of the locking device. Thus, work machine 1 achieves operating point "T". The differential system then either operates in this operating point or connects the differential drive 5 to the power supply system. In this case, a rotational speed, which is derived from the selected/preset number of pole pairs or the direction of the rotating magnetic field at the differential drive 5, then occurs at the differential drive 5. The operating point set for the work machine 1 is then determined as a function of the direction or pole pair number of the rotating magnetic field selected for the differential drive 5 and the transmission ratio of the differential drive 3 and the matching transmission stage 10.

However, the system may of course also increase the power in any other way, for example by: the drive 4 and the differential drive 5 are connected to the power grid simultaneously or the differential drive 5 and then the drive 4 are connected to the power grid. In the case of non-electrical drives, for example, the differential drive 5 can also be connected upstream of the drive, simultaneously with the drive and downstream of the drive.

The work machine 1 can be operated in this configuration, i.e., without a frequency converter, with a variable rotational speed in a discontinuous manner. When work machine 1 is, for example, a conveying device in a line system, a throttle or cover plate or bypass valve or valve can be arranged in the line system downstream of work machine 1. The flow rate can thus be adjusted, if necessary, between a likewise fixed flow rate or delivery height set on the basis of a fixed rotational speed of the differential system.

For high device availability, the following advantages are significant: the entire system can continue to operate without interruption in the event of a failure of the

frequency converters

6a, 6 b. However, the following boundary conditions are noted here:

this is the system behavior, in particular the system operating range between operating points "B" and "C", in which the differential drive 5 operates at high rotational speeds in a generator-like manner. If the

frequency converters

6a, 6b fail in the operating range, the differential drive 5 accelerates instantaneously and there is a risk of reaching a harmful overrun range.

This can be prevented by the operating system according to the invention in that: preferably, the

brakes

13, 14 or any other reduction means which act on the rotational speed of the second drive are activated so quickly that they do not necessarily stop the second drive but prevent an overspeed of the differential system or of the second drive of the differential drive 5 which is detrimental to the system.

If it is desired to reach the operating point "T", the second drive of the differential system is braked.

If one of the operating points "B" or "a" is to be reached, the differential drive 5 is connected directly to the electrical grid 12 in a further, preferably parallel step. If this occurs fast enough, the activation of one of the above-described reduction means can be dispensed with. Furthermore, if the possibility of reaching operating point "B" is omitted, for example, a commutation protection circuit is saved and at the same time either operating point "T" or alternatively operating point "a" is preferably reached, in that: the differential drive 5 is connected to the electrical network 12 in the direction of rotation of the standard rotating magnetic field required for this purpose.

If operating point "B" is reached, the reduction device can also be used for network synchronization of the rotational speed regulation of the differential drive 5, by: the reduction gear is activated in such a way that the synchronous rotational speed of the differential drive is set substantially at the differential drive 5 before the differential drive is connected to the electrical grid 12.

In principle, a continuously variable rotational speed operation can also be achieved with the reduction gear in the operating range of the system between operating points "C" and "T". This is for example expedient if the differential drive 5 fails or if the power of the differential drive 5 or of the

inverters

6a, 6b is insufficient to provide the required operating torque.

Fig. 5 shows a characteristic curve of a three-phase electric machine (which can be used, for example, as a differential drive 5), in which the rotating magnetic field is connected to the power grid in such a way that the operating point "a" (the "rated point" of the three-phase electric machine in fig. 5) is set according to fig. 4. In the system operating range between points "C" and "T", the three-phase machine moves along the dotted section of the characteristic curve ("reverse-current braking range"). As can be seen from the representation in fig. 5, the torques (M) that can typically be achieved by the three-phase machine in the reverse current braking range_i/M_iK) Significantly less than the rated torque of the three-phase motor. However, the torque in the reverse current braking range can be increased by a special design of the rotor shaft of, for example, a three-phase motor. If this is not sufficient to set the differential system to the desired operating point with a fixed rotational speed, the reduction device, for example the brake already described, is preferably additionally activated. It is also preferred that the differential drive 5 is in a frequency converterIn the system operating range above "T" (range of motor (+) in the event of a

fault

6a, 6b) the range of generator (-) according to the characteristic curve of fig. 4 is reached. In both cases, the reduction gear is preferably activated for as long as, on the one hand, the differential drive 5 is directly connected to the grid and, on the other hand, the system operating speed is in the range of the generator type (-) of the characteristic curve(s) ((<"T") motion.

The possible operating points with a fixed rotational speed at which the work machine 1 can achieve a reduced power transmission between operating points with a fixed rotational speed without the

frequency converters

6a, 6b are set by means of a throttle device. The throttle device, which may be positioned downstream of work machine 1, for example, in the line system, may adjust a throttle, a cover plate, a bypass valve, or a valve, for example. When the work machine 1 is, for example, a mechanical drive, the throttle device can be, for example, a brake, a retarder or the like, in order to reduce the power of the drive 1.

In this case, however, only operating points "below" (in the direction of a lower rotational speed or delivery quantity or power) the respective operating point at which the rotational speed is fixed (fixed) are reached. In order to achieve an optimum efficiency of the entire operating range of the system, the operating requirements can be varied between the fixed rotational speed points according to the invention. In order to operate the entire system as efficiently as possible, a fixed operating point ("B", "T" or "a") at a higher speed (in the direction of a higher speed or throughput) is preferably always set by the differential drive 5 and reached by means of the throttle device.

In a preferred embodiment according to the invention, however, in order to set the flow rate and the delivery height as required, depending on the operation, the rotational speed is changed between fixed operating points "a", "T" and "B", without the use of a throttle valve.

If the system is in an operating point between "B" and "T" at the time of failure of the frequency converters 6a, 6B, the differential drive 5 is preferably either switched directly on to the electrical network 12, so that the operating point "a" is reached, or the operating point "T" is set by braking a second drive of the differential system.

If the system is in an operating point between "T" and "a" at the moment of failure of the

frequency converters

6a, 6b, the differential drive 5 is preferably switched directly on to the power grid 12 in such a way that the operating point "a" is reached directly without having to activate the reduction of the second drive of the differential system (if the system operating speed, as described above, for example, does not drop below "T").

According to the invention, the described control concept with fixed rotational speed operating points "a", "T" and "B" can also be extended to systems with switchable differential drives 5, whereby a correspondingly greater number of fixed rotational speed operating points are obtained, between which an optimum change in operation is then possible.

Claims

1. Method for operating a drive system comprising a drive shaft (2), a drive (4), and a differential (3) having a first drive, a second drive, and a driven gear, wherein the driven gear is connected to the drive shaft (2), the first drive is connected to the drive (4), and the second drive is connected to an electrical differential drive (5) which can be connected to an electrical network (12) via a frequency converter (6a, 6b), wherein the drive shaft (2) can be operated at an operating point with a fixed rotational speed,

characterized in that the differential drive (5) is operated via a frequency converter (6a, 6b), or in that the drive shaft (2) is driven by the electrical differential drive (5) at synchronous rotational speed during the time when the differential drive is connected to the electrical network (12) via a line connected in parallel with the frequency converter (6a, 6b) either directly or only via a transformer (11),

the driving shaft (2) is connected with a working machine (1) and provides a plurality of operating points with fixed rotating speed for the working machine (1), and the mode is as follows:

-when the second drive of the differential (3) is stationary, the working machine is operated at a base speed (T) at a fixed speed operating point; or

-operating the work machine at one or more operating points with a fixed rotational speed by means of a pole-switchable differential drive (5); or

-operating the work machine at one or more operating points with a fixed rotational speed by reversing the direction of the rotating magnetic field of the differential drive (5),

and, in the event of a failure of the frequency converters (6a, 6b), the working machine (1) is operated at a higher operating point with a fixed rotational speed.

2. Method according to claim 1, characterized in that the differential drive (5) is connected to the electricity network (12) via a conductor (15, 18) when the frequency converter (6a, 6b) has a fault.

3. Method according to claim 1, characterized in that the second drive is a sun wheel (9).

4. A method according to any one of claims 1 to 3, wherein a transition is made between the operating points during operation.

5. A method according to any one of claims 1-3, characterised in that the second drive is braked by means of a reduction unit, which acts on the second drive, when the frequency converter (6a, 6b) fails and the power machine (1) is operating below the basic speed (T).

6. Method according to claim 5, characterized in that the reduction gear is a force-and/or form-locking brake (13, 14), retarder or differential lock.

7. A method according to claim 5, characterized in that the reduction means are activated during operation of the differential drive (5) below the basic speed (T) in the event of a failure of the frequency converter (6a, 6b) until one of a higher speed-fixed operating point or a higher speed-fixed operating point is reached, however not exceeding the basic speed (T) at the maximum.

8. Method according to one of claims 1 to 3, characterized in that the power delivered by the work machine (1) is reduced by means of a throttle device when operating at an operating point with a fixed rotational speed.